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Souped-Up Pulses Top Light Speed

It isn't quite warp drive, but scientists have managed to make light go, well, faster than the speed of light. Thanks to a clever trick, a beam of microwaves travels faster than the ultimate speed limit, but without breaking the law of Einstein's theory of relativity.

One of the bulwarks of Einstein's theory is that the speed of light in a vacuum, c, is an inviolable limit; there is no way you can transmit information faster than c. Researchers have devised some seeming violations of the rule, but these have been limited to artificial conditions such as pulses traveling very short distances through thin barriers.

Now physicist Anedio Ranfagni and colleagues at the Italian National Research Council in Firenze have demonstrated a method for getting superluminal velocities in free space rather than through a barrier. It's actually geometric sleight-of-hand. They pump a beam of microwaves through a centimeters-wide ring and reflect it off of a curved mirror to create what's called a "Bessel beam" of microwave light. Viewed from above, a Bessel beam has two planes of waves that intersect like an "X." And just as the point of intersection of a pair of scissors can seem to move much faster than the individual blades, the intersection of the two plane waves can move faster than the speed of the plane waves themselves. Rafagni's team found that the location of the Bessel beam's intersection did indeed travel as much as 7% faster than the speed of light, they report in the 22 May issue of Physical Review Letters.

However, this doesn't violate Einstein's theory, explains Aephraim Steinberg, a University of Toronto physicist, because the moving intersection can't carry a signal. Nonetheless, Steinberg hopes that such superluminal velocities might reduce delays in electrical circuits, although he admits "it seems a little far out at the moment."